According to the approach made famous by Alchourrón, Gärdenfors and Makinson (1985), revision is a transformation K*h of a potential belief state K by adding h yielding another potential belief state.1 This AGM revision transformation is a composition of two other transformations: contraction and expansion. K*h = [K-~h]+h. This is the expansion by adding h of the contraction K-~h of K by removing ~h.